Study of the Influence of Nickel Content and Reaction Temperature on Glycerol Steam Reforming with N

International Journal of Research in Engineering and Science (IJRES) ISSN (Online): 2320-9364, ISSN (Print): 2320-9356 www.ijres.org Volume 5 Issue 1 ǁ January. 2017 ǁ PP.54-62

Study of the Influence of Nickel Content and Reaction Temperature on Glycerol Steam Reforming with Ni/La2O3-Sio2 Catalysts Vivian Vazquez Thyssen1; Francisco Guilherme Esteves Nogueira2; Elisabete Moreira Assaf3* 1,3

2

(Instituto De Química De São Carlos - Universidade De São Paulo - São Carlos/SP - Brazil) (Departamento De Engenharia Química - Universidade Federal De São Carlos - São Carlos/SP - Brazil)

ABSTRACT: La2O3-SiO2-supported nickel catalysts were evaluated in glycerol steam reforming. The samples (30wt% La and 5, 10 and 15wt% of Ni on 70wt% commercial SiO 2), prepared by the simultaneous impregnation method, were characterized by EDX, nitrogen physisorption, XRD, in-situ XRD, XANES and TPR. The analyses revealed NiO species weakly interact with the support and the different metallic surface areas of the catalysts. Catalytic tests were performed in a fixed bed reactor at 600oC and 15Ni catalyst, which showed the best performance, was also evaluated at 500oC and 700oC. According to the results, the Ni content on the catalyst surface interferes in the distribution of gaseous products H2, CO, CO2 and CH4. The increase in the Ni content increases the carbon formation during reaction. The reaction temperature affected the catalytic performance and the best results were obtained with the 15Ni catalyst at 600oC, which was also tested for 20 hours for the analysis of its stability.

Keywords: catalysts; nickel content; steam reforming; glycerol; reaction temperature

I. INTRODUCTION Most of the world’s energy demand is supplied by fossil fuels, which seriously endanger the environment – e.g. increased global warming. Therefore, the use of alternative, clean and sustainable energy sources is vital for the future. The production of H2 as a fuel has been studied for decades and the most popular methods involve the steam reform of hydrocarbons and alcohols.

Glycerol is produced as a byproduct in the transesterification reaction for the obtaining of biodiesel (Reaction 1), which has been increasingly used as a fuel. Therefore, the extending biodiesel production results in a large supply of glycerol in the market.

C3H5(OOC)3(Rn)3 + 3R’OH  3RnCOOR’ + C3H5(OH)3

(Reaction 1)

The excessive amount of glycerol in the market can be used in the H2 production, since 1 mol of glycerol can produce 7 mols of H2 from its steam reforming reaction (Reactions 2-4).

C3H5(OH)3  3CO + 4H2 3CO + 3H2O  3CO2 + 3H2 C3H5(OH)3 + 3H2O  3CO2 + 7H2

(Reaction 2) (Reaction 3) (Reaction 4)

The catalytic conversion of glycerol into H2, CO2 and CO involves the preferential cleavage of C-C bonds as opposed to breaking C-O bonds. Studies have suggested catalysts containing Ni favor the breaking of the C-C bond of alcohols for the formation of CH4, CO2, CO and H2. The increase of the Ni content in the catalyst improves such a property. [1-3] SiO2 is used as a support in reforming reactions due to its high surface area [4] and La2O3 used as an additive is known for its ability to remove the carbon formed during the reaction [3].

Many authors have studied the glycerol steam reforming reaction at different temperatures and observed the conversion of alcohol increases in function of the temperature, whereas the amount of carbon and volume of the liquid effluent collected at the end of the reaction decrease. [5,6] In previous studies [3,7], we discussed the effect of La2O3 content on the support and the effect of the material preparation method. In Thyssen et al. (2013) [3], the best results were achieved with the use of 30wt% La2O3 content. Thyssen and Assaf (2014) [7] observed the preparation method of simultaneous impregnation was more effective. Therefore, this paper reports the study of Ni catalysts with different amounts of metal, supported on 30wt%La2O3-SiO2 prepared by simultaneous impregnation, to be used for the obtaining of H2 from glycerol steam reforming at different temperatures. www.ijres.org

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Study Of The Influence Of Nickel Content And Reaction Temperature On Glycerol Steam …

II. EXPERIMENTAL 2.1 Preparation Catalysts were prepared by the simultaneous impregnation method, as described by Thyssen and Assaf (2014) [7], with 5, 10 and 15wt% of Ni deposited on a support composed of 30wt%La2O3-70wt%SiO2 and selected according to Thyssen et al. (2013) [3].

Commercial SiO2 (Degussa - Aerosil 200) was treated at 600oC for 2h under synthetic air flow. La(NO3)3.6H2O and Ni(NO3)2.6H2O aqueous solutions were simultaneously impregnated in the commercial SiO2 and calcined at 600oC for 3h, also under synthetic air flow. The catalysts were labeled 5Ni, 10Ni and 15Ni. 2.2 Characterization Energy-dispersive X-ray (EDX) spectroscopy determined the chemical composition of the catalysts placed under an LEO 440 electron microscope (Leica Zeiss) coupled to an energy-dispersive analyzer (SiLi detector) with a beryllium window (Oxford 7060) and 133eV resolution.

Surface specific areas of the samples were determined by N2 physisorption (BET method) in a Quantachrome Nova 1000e instrument.

The samples were characterized by X-ray diffraction (XRD) for the identification of the crystalline phases. XRD analyses were performed with CuKα radiation on a Rigaku Multiflex diffractometer. Bragg angle (2θ) was scanned between 10o and 80o at 2omin-1 and Scherrer’s equation (Equation 1) calculated the NiO crystallite size.

d NiO 

λ. 2 β.cosθ

(Equation 1)

where dNiO is the NiO crystallite average size (nm), λ is the incident radiation wavelength (nm), β is the halfheight width (radians) of the NiO main peak and θ is the Bragg angle of the peak (o).

XRD analyses were recorded in situ under activation conditions at the Brazilian Synchrotron Light Laboratory-LNLS (Campinas, Brazil) on XPD-10B beamline and in reflection mode, in a 2 interval from 40o to 80o, at λ = 1.54996Å radiation, calibrated with a Si (1 1 1) monochromator. The samples were heated, in situ, under H2, from 25oC up to the reduction temperature, at 10oCmin-1, and held at that temperature for 1h. The crystallite size of Ni0 (dNi) was calculated by Equation 1 from in situ XRD patterns.

The metal dispersion of the catalysts (DM) was calculated by Equation 2 from the crystallite size of Ni0.

The metallic surface area was obtained by Equation 3, where M is the Ni metal atomic weight, N is the Avogadro number and y is the Ni mass fraction assuming a density of active sites on the surface (ns) 1.54x1019 Ni atoms.m-2 [8].

DM 

101.2 d Ni

(Equation 2)

SM 

D M .N.y n s .M

(Equation 3)

2.3 Catalytic tests Glycerol steam reforming (GSR) tests were performed as described by Thyssen et al. (2013) [3], at a 3:1 molar water:glycerol ratio, 2.5mLh-1 flow rate and 150mg of catalyst. They were conducted at 600oC for 5h for all catalysts and at 500oC and 700oC for the catalyst with 15wt% of Ni, which was also tested for 20h for the analysis of its stability at 600oC. Prior to the reaction, the catalysts were activated in situ for 1h under an H2 flow at 30mLmin-1, at determined TPR temperature profiles (400oC, 600oC and 500oC; for 5Ni, 10Ni and 15Ni, respectively).

The collected effluent was analyzed by gas chromatography in a Shimadzu system with H2 carrier gas and an HP5 capillary column operating between 35oC and 250oC, with an FID detector. The global glycerol conversion (XGlycerol) was calculated by Equation 4.

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Study Of The Influence Of Nickel Content And Reaction Temperature On Glycerol Steam …

n  n residual  .100 X glycerol   total n total  

(Equation 4)

where ntotal is the amount of glycerol fed (mol).

A gas chromatograph (Varian GC-3800) was used in-line for the analysis of the gaseous products reaction (H2, CO, CO2 and CH4). It was equipped with two parallel columns (Porapak-N and Molecular Sieve 13X), with TCD in each of them, operating between 40oC and 80oC, and He and N2 carrier gases, respectively, at 10mLmin-1. The yield (Y) for the gaseous products was determined according to the ratio of moles of the product formed and moles of the glycerol fed (Equation 5).

Y

mol prod

(Equation 5)

mol glycerolfed

The qualitative analysis of the accumulated liquid products of the reaction was performed by a GCMSQP20105 system (Shimadzu) equipped with an rtx-wax column (30m x 1μm x 0.32mm) after the end of the reaction. The column temperature ranged from 40oC to 200oC and a 280oC injector temperature was set.

An EA1110 CHNS-O Elemental Analyzer equipped with a Porapack PQS column determined the carbon formed throughout the reaction.

A scanning electron microscopic (SEM) analysis of the 15Ni catalyst used was performed under an LEO-440 scanning electron microscope equipped with an Oxford detector.

III. RESULTS AND DISCUSSION 3.1 Characterization The mass compositions of the catalysts, obtained by EDS, are shown in Table 1.

Table 1. Catalysts physicochemical properties.

Catalysts

5Ni 10Ni 15Ni a

Content (wt%) La 33.1±2.9 33.5±1.8a 31.8±1.6

S (m2g-1)

Ni 4.6±1.2 9.8±0.7a 16.8±1.9

129 121a 97

Crystallite size (nm) NiO Ni0 8 34 11 20 13 20

TPR-H2 consumption (x10-5mol) 1.0 9.6a 14.1

R (%)

DM (%)

7 100a 100

3 5 5

SM (m2g1 ) 1.0 3.3 5.0

Data based on Thyssen and Assaf, 2014 [7]

Results for La and Ni were close to those expected, i.e., 30wt% for La(III) and 5, 10 and 15wt% for Ni(II). Slight differences may be due to the manipulation of reagents during the preparation.

Table 1 also shows the BET surface area for catalysts with different Ni(II) contents. According to Guo et al. (2007) [9] and Dieuzeide et al. (2014) [10], the surface area of catalysts with different Ni(II) contents decreases, as the percentage of Ni(II) in the catalyst increases, which was also observed here.

All three catalysts showed the same crystallographic phases in the diffractograms (Figure 1a): besides the structures of the supports, SiO2 (JCPDS 83-2473) and La2O3 (JCPDS 83-1344), only the NiO crystalline phase was observed (JCPDS 78-0643). [3,7] Figure 1. (a) XRD of catalysts, (b) XRD in situ of catalysts after activation with H2

a

Data based on Thyssen and Assaf, 20147 www.ijres.org

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Study Of The Influence Of Nickel Content And Reaction Temperature On Glycerol Steam ‌ A low and broad peak is expected for amorphous materials, as they display no regular crystal planes or a long-range structure. [3,7] As shown in Figure 1a, the crystallinity of SiO2 and La2O3 seems to be low.

The difference in the intensity of the NiO peaks may be correlated with the amount of Ni(II) species.

An increase in the Ni(II) content of the catalyst increases the intensity of the peaks, which suggests the amount of NiO species is higher in catalysts with a higher Ni(II) content. [11,12] The NiO average crystallite size (Table 1) was estimated by Scherrer’s equation (Equation 1) from XRD patterns. It showed a small increase when the Ni(II) content in the catalyst was increased. Li et al. (2012) [13] reported no remarkable NiO particle size change was observed at metal loading below 16wt%.

The in situ XRD (Figure 1b) after the activation process revealed the 10Ni and 15Ni catalysts exhibited only Ni0 peaks (JCPDS 4-850), whereas 5Ni also showed peaks related to the presence of NiO. The results confirmed the calculus of the reduction degree of the samples (R - Table 1), i.e., all NiO phases in the 10Ni and 15Ni catalysts were reduced to metallic Ni during the reduction process, whereas the 5Ni catalyst showed only a 7% reduction [14]. The Ni0 average crystallite size is also provided in Table 1.

The metallic dispersion (DM) of the catalysts (Table 1) was calculated from Ni0 crystallite size and their metallic areas were calculated from DM results. Table 1 shows both dispersion and metal areas are larger for smaller Ni0 crystallite sizes.

Figure 2 shows the XANES results for the catalysts. The spectral features for all catalysts calcined at 600oC are close to those displayed by the NiO reference spectrum, which leads to the hypothesis the Ni local chemical environment is similar, regardless of the Ni content.

Figure 2. XANES of catalysts (a) 5Ni, (b) 10Ni and (c) 15Ni

The XANES results, along with XRD results, suggest La2O3 does not play any meaningful role in the local chemical environment of Ni, which occasionally would form LaNiO 3 wherein Ni would be more strongly interacted with the supporting La2O3, as observed in previous studies [3,7]. Figure 2 shows the LaNiO3 species are not present in those catalysts.

The TPR profiles of the catalysts are displayed in Figure 3. The catalysts showed a reduction peak around 350-500oC, which represents an Ni-support interaction. According to the XRD results in Figure 1a, the reduction peak in the TPR profiles may be related to the reduction of NiO that weakly interacts with the support.

Due to its high mobility on the surface of the material, which leads to an easy migration and possible aggregation of particles, NiO is easily reduced and described as a free state of the Ni active phase, as explained in Thyssen et al. (2013) [3] and Thyssen and Assaf (2014) [7]. www.ijres.org

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Study Of The Influence Of Nickel Content And Reaction Temperature On Glycerol Steam ‌ Figure 3. TPR of catalysts

a

Data based on Thyssen and Assaf, 20147

The easy migration of NiO particles may explain the low reduction degree of 5Ni (Table 1). Since NiO may be aggregated on the 5Ni surface, its reduction may be difficult.

According to Table 1, the amount of TPR-H2 consumption increased according to the increase in the Ni(II) content of the catalyst due to the increase in the absolute amount of Ni(II) species. However, such an increase is not proportional to the Ni content, since the amount of Ni in the 5Ni catalyst is much lower than half the amount of 10Ni [15], which may corroborate the aggregation of NiO particles on the 5Ni surface.

In the simultaneous impregnation method, both lanthanum and nickel oxides compete for adsorption sites on silica, as explained in Thyssen and Assaf (2014) [7]. Therefore, for those catalysts, neither a homogeneous surface, nor the presence of Ni(II) species in the bound state are expected – which suggests a stronger interaction with the support. And this may be due to La2O3 is not uniformly dispersed over the SiO2 before the Ni(II) adsorption takes place [7]. The XANES results show La2O3 does not play any meaningful role in the local chemical environment of Ni in those catalysts, and since a homogeneous catalyst surface is not expected and NiO is easily aggregated, the NiO reduction is not equal.

3.2 Catalytic tests Table 2 shows the GSR results for catalysts with different Ni contents.

Table 2. C gaseous product yield (molprod/molglycerol fed), H2/CO2 and CO2/CO rate (mol/mol), volume of liquid effluent collected (mL), global glycerol conversion at 600 oC (%) and carbon formation (mmol) Catalysts 5Ni 10Nia 15Ni 15Ni - 20h

H2 2.0 3.8 4.6 4.2

CH4 0.1 0.4 0.5 0.4 a

CO 0.3 1.1 1.2 1.0

CO2 0.7 1.5 1.8 1.4

H2/CO2 2.9 2.5 2.6 2.9

CO2/CO 2.3 1.4 1.5 1.4

Effluent 5.8 3.8 3.3 14.7

Xglycerol 23 98 100 100

C 4.0 5.0 6.0 22.0

Data based on Thyssen and Assaf, 2014 [7]

The amount of collected liquid is related to the global glycerol conversion, i.e., the higher the amount of the liquid, the lower the global conversion of glycerol reached by the catalyst. The collected effluent was analyzed and acetic acid, propanoic acid, formic acid, ethanol, 2-propanone, propylene glycol, glycidol, 1,3dioxane and sorbitol were identified.

Metallic Ni is the active phase for GSR to take place, which favors the breaking of the C-C bond.

Therefore, the more dispersed the Ni (or the higher the metal surface area), the more active the catalyst. 15Ni catalyst showed the highest activity in the GSR and the largest metallic area.

The catalyst with lowest nickel content (5Ni) showed low GSR activity, which might be explained by the effect of the low metallic area. The products expected during the reaction (H2, CH4, CO and CO2) showed www.ijres.org

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Study Of The Influence Of Nickel Content And Reaction Temperature On Glycerol Steam … low yield in comparison with the other two catalysts (Figure 4) and also a low global conversion of glycerol.

Since the small amount of converted glycerol was not efficiently converted to the expected gaseous product, it would not be efficient for the process.

By GSR reaction (Reaction 4), the theoretical H2/CO2 ratio is approximately 2.3mol/mol. The ideal value for the CO2/CO ratio is 1mol/mol, since the shift reaction (Reaction 3), in which the CO formed during the reaction is converted to CO2 and H2, shows an equilibrium and would lead to 1mol/mol CO 2/CO. According to Table 2, the H2/CO2 ratio of the three catalysts was larger or very close to the theoretical one (2.3mol/mol) and the CO2/CO ratio was larger than the theoretical one (1mol/mol), which shows a higher selectivity for CO 2 than for CO in those catalysts.

10Ni and 15Ni catalysts yielded very similar carbonaceous gaseous products (CH4, CO and CO2 – Table 2) – only the H2 yield showed an improvement with the increase in the Ni content – and both catalysts were active and stable over the entire reaction (Figure 4).

The amount of carbon produced showed a small difference among the catalysts (Table 2) with a slight increase in function of the increase in the Ni content.

The literature [16,17] reports catalysts with Ni as an active phase promoting the formation of carbon as filament, i.e., carbon is deposited between the metal and the supporting surface and forms a filament that can be broken, which results in the deactivation of the catalyst with the loss of the active phase.

Figura 4. GSR of (a) 5Ni, (b) 10Ni and (c) 15Ni (■H2 ●CH4 ▲CO ▼CO2)

The dissociation reaction of CH4 (Reaction 5) and Boudouard reaction (Reaction 6) may be responsible for the formation of carbon during the catalytic test.

CH4  2H2 + C 2CO  CO2 + C

(Reaction 5) (Reaction 6)

The Boudouard reaction (Reaction 6) should be considered over Ni catalysts, since the presence of nickel species in the form of crystallites – observed in the catalyst characterization – favors the carbon atom diffusion [18]. The increase in the Ni content may increase the amount of Ni on the material surface, therefore, an increase in the carbon deposition at the end of the reaction is also expected. www.ijres.org

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Study Of The Influence Of Nickel Content And Reaction Temperature On Glycerol Steam …

Despite such a slight increase in the carbon formation with the increase in the Ni content, the 15Ni catalyst showed an excellent H2 yield and 100% glycerol conversion and its 20-h stability test (Table 2 and Figure 5) showed it was stable and active during all process.

The carbon formation for the 15Ni (Table 2) catalyst was equivalent regarding time, since the carbon deposition rate for the catalyst was 1.1mmolh-1 in a 20-h reaction and 1.2mmolh-1 in a 5-h reaction.

Figura 5. GSR of 15Ni at 600oC for 20h (■H2 ●CH4 ▲CO ▼CO2)

Figure 6 shows the SEM image of the 15Ni catalyst after a 20-h stability test at 600oC and the formation of filamentous carbon over the reaction, as expected. [19-21] Figure 6. SEM of 15Ni after GSR

Table 3 shows the results of the 15Ni catalyst GSR for 5h at different temperatures. According to Figure 7, the 15Ni catalysts were stable at both temperatures evaluated.

Table 3. 15Ni C gaseous product yield (molprod/molglycerol fed), H2/CO2 and CO2/CO rate (mol/mol), volume of liquid effluent collected (mL), global glycerol conversion (%) in 5-h reaction at 500oC, 600oC and 700oC, and carbon formation (mmol) 15Ni 500oC 600oCa 700oC a

H2 2.4 4.6 4.1

CH4 0.4 0.5 0.1

CO 0.2 1.2 1.7

CO2 0.8 1.8 1.1

H2/CO2 3.0 2.6 3.7

CO2/CO 4.0 1.5 0.6

Effluent 4.9 3.3 3.1

Xglycerol 81 100 100

C 9.5 6.0 5.0

Data based on Table 2 www.ijres.org

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Study Of The Influence Of Nickel Content And Reaction Temperature On Glycerol Steam …

Figure 7. GSR of 15Ni at (a) 500oC and (b) 700oC (■H2 ●CH4 ▲CO ▼CO2)

An increase in the temperature promotes an increase in the glycerol conversion, whereas the liquid effluent volume collected at the end of the reaction decreases (Table 3).

The H2/CO2 ratio of the 15Ni catalyst was higher than the theoretical one (2.3mol/mol) at all temperatures studied, therefore, the selectivity in H2 is higher than the selectivity in CO2 at all temperatures.

The CO2/CO ratio was far higher than the theoretical one (1mol/mol) at 500oC, close to the theoretical one at 600oC and below 1 at 700oC. Through the shift reaction (Reaction 3), the CO formed during the steam reforming reaction is converted into CO2 and H2. The reaction is reversible and its equilibrium is shifted to the right at lower temperatures. However, at higher temperatures, the reaction equilibrium is shifted to the left, which limits the conversion of CO into CO2. Therefore, the selectivity of CO increases with increasing temperature.

The temperature range can influence the formation of carbon in steam reforming reactions and the increase in temperature causes a reduction in the carbon deposition during the reaction. Since the Boudouard reaction (Reaction 6) is an exothermic reaction, as the 'shift' reaction (Reaction 3), those reactions are less favored at elevated temperatures. Therefore, the carbon formation from the CO consumption is disfavored at 700oC, which may also explain the higher selectivity in CO at that temperature.

IX. CONCLUSIONS The amount of Ni in the catalyst affects the selectivity in H2, CO, CO2 and CH4 due to the Ni property of cleaving the C-C bond. Furthermore, 10wt% Ni and 15wt% Ni catalysts showed better performance than the catalyst with 5wt% Ni. An increase in the Ni content in the catalyst promoted a slight increase in the carbon formation during the reaction, which can be associated with the Boudouard reaction favored in the NiO presence.

The 15Ni catalyst was also evaluated for 20h and its behavior was equivalent to that obtained by a 5hour reaction active throughout the reaction. A scanning electron microscopic analysis conducted in the catalyst after the stability reaction confirmed the formation of filamentous carbon.

The 15Ni catalyst was also tested at 500oC and 700o and we have concluded temperature is an important factor for the glycerol steam reforming reaction, since some reactions that take place in parallel with glycerol steam reforming are favored or do not depend on temperature. The catalyst shows better performance at 600oC.

ACKNOWLEDGMENTS The authors acknowledge CAPES, CNPq and FAPESP for the financial support, Electrochemistry Group at IQSC for the XRD analyses and LNLS (Campinas, Brazil) for the in situ XRD and XANES analyses.

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